Income Taxes (Reconciliation of Income Taxes at the U.S. Statutory Federal Income Tax Rate) (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
U.S. statutory rate	21.00%	35.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	4.10%	2.80%
Investment tax credits	(0.50%)	(4.20%)
Production tax credits	(0.60%)	(0.80%)
Reversal of excess deferred income taxes	(1.40%)	(0.00%)
Other, net	(2.10%)	(3.80%)
Effective tax rate	20.50%	29.00%
Virginia Electric and Power Company
Income Tax Contingency [Line Items]
U.S. statutory rate	21.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	4.50%	3.80%
Investment tax credits	(0.90%)	(0.80%)
Production tax credits	(0.70%)	(0.60%)
Reversal of excess deferred income taxes	(2.00%)	(0.00%)
Other, net	(0.30%)	(0.60%)
Effective tax rate	21.60%	36.80%
Dominion Energy Gas Holdings, LLC
Income Tax Contingency [Line Items]
U.S. statutory rate	21.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	4.30%	0.30%
Investment tax credits	(0.00%)	(0.00%)
Production tax credits	(0.00%)	(0.00%)
Reversal of excess deferred income taxes	(1.00%)	(0.00%)
Other, net	(0.40%)	(0.60%)
Effective tax rate	23.90%	34.70%